Solar cell

ABSTRACT

A solar cell includes: a passivating layer sequence having a plurality of island-like recesses from which the passivating layers have been completely removed; a thin first metal layer situated on the passivating layer sequence and situated in the recesses on the substrate surface; a thin dielectric cover layer covering the first metal layer which has a first regular arrangement of narrow line-type openings and a second regular arrangement of essentially wider line-type or extended island-type openings, the first and second opening arrangement being aligned at an angle; and a highly conductive second metal layer able to be soldered at the exposed surface in the openings of the first and second opening arrangement and contacts the first metal layer there.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to the domain of semiconductor components,especially solar cells, and relates to a crystalline solar cell havingan n- or p-doped semiconductor substrate and a passivated backside.

2. Description of the Related Art

Silicon solar cells having a “passivated backside” have improved opticalaluminizing and a greatly improved passivation of the back surfacecompared to the aluminum back surface field (BSF) that has been producedin a standard manner up to now, cf. A Götzbarger et al., “Sonnenenergie:Photovoltaik” (Solar Energy: Photovoltaics), B. G. Teubner Stuttgart,1997. The cell concept produced thereby is called “Passivated Emitterand Rear Cell” (PERC). In this context, the dielectric passivationadapted to the backside doping is opened locally at many small points,so that the metal layer deposited on the passivating layer is not ableto contact the semiconductor, but only at a small surface portion of thebackside, in order to minimize the strong recombination of the electronhole pairs at metallized surfaces.

The metallizing of the backside is made up in most cases of aluminum,and is deposited over a large surface on the entire backside, as a rule,using vacuum vapor deposition technology or sputtering.

Documents relating to such solar cells as well as methods for theirproduction, and are connected to backside patterning steps and/or thebackside driving in of doping substances, are the likes of publishedGerman patent document DE 195 25 720 C2, published German patentapplication document DE 10 2007 059 486 A1 or published German patentapplication document DE 10 2008 013 446 A1, as well as published Germanpatent application document DE 10 2008 033 169 A1 (both of the latterfrom ErSol Solar Energy AG).

Such documents as published Japanese patent application document JP 2005027 309 A, DE 10 2008 017 312 A1 or published German patent applicationdocument DE 10 2008 020 796 A1 are concerned with the efficientproduction of reliable connecting patterns, particularly those havingsoldered connections.

The method “laser fired contacts” (LFC) is known, in which themetallization is fired through on the backside passivation using laserpulses, so that a prior opening of the passivating layer becomesunnecessary, cf. “Laserstrahlverfahren zur Fertigung kristallinerSilizium-Solarzellen” (Laser Beam Method for Producing CrystallineSilicon Solar Cells), Dissertation by Eric Schneiderlöchner,Albert-Ludwigs-Universität Freiburg im Breisgau (2004) or (earlier)published German patent application document DE 199 15 666 A1.

In the case of p-doped wafers, the formation of “local BSF regions” atthe contact points in the backside passivation is undertaken simply byalloying the aluminum into the p− or p⁺ surface. This takes place attemperatures above the Al—Si eutectic temperature of 577° C. For this itis necessary that the backside passivation layer and also the front sideemitter (after the sintering of the front side silver paste) survivethese temperatures undamaged.

In the case of n-doped wafers, in which the emitter (p-n junction) isproduced on the backside using boron or aluminum doping, the formationof the Al—Si eutectic, which, as a rule, would melt a few micrometers indepth, would lead to a breakthrough through the thin p⁺ emitter into then-base and there it would lead to a short circuit to the base. For thatreason, the metallization has to be tempered at low temperatures (e.g.400° C., optionally also in forming gas), in order to produce asufficiently good ohmic contact to the emitter surface, without damagingthe cell. This makes impossible the firing of a normal screen printingpaste for the subsequent depositing of a silver layer, that is able tobe soldered, on the aluminum.

All the PERC technologies, known from the related art, have thefollowing disadvantages:

-   1.) It is common to all the concepts that the backside is present at    the end of the process having a large-surface aluminum layer, which    does not have solderable pad areas.-   2.) The low ohmicity of the metallization required for the current    conduction is only able to be produced via a sufficiently thick    aluminum layer, as a rule, 2-4 μm. If the vapor deposition or    sputter rate is selected to be high, in order to keep the processing    times low, the heat input, and with that, the temperature of the    wafer in the process chamber, increase greatly. If the rate is held    to be low, more time or a longer passage system having additional    deposit or sputtering sources for the aluminum coating have to be    provided. In any case, the compromise is connected to higher    investment and or processing costs.-   3.) One alternative is a thinner aluminum layer which is able to be    produced in a sufficiently short period of time at a moderate    deposition rate, but then a chemical or galvanic reinforcement of    the Al backside metallization is required. This takes place, though,    at very moderate temperatures (<90° C.) and thus represents no    thermal stress of the almost ready solar cells.-   4.) If one found a suitable method for reinforcing aluminum    chemically or galvanically, however, the entire backside would be    reinforced with silver, which would represent a considerable cost    factor.

BRIEF SUMMARY OF THE INVENTION

The solar cell proposed is distinguished by a thin dielectric coverlayer covering the first metal layer, which has a first regulararrangement of narrow line-type openings and a second regulararrangement of essentially wider line-type or extended island-typeopenings, the first and second opening arrangement being aligned at anangle, particularly transversely to each other. It is distinguishedfurther by a highly conductive second metal layer that is able to besoldered at the exposed surface in the openings of the first and secondopening arrangement which contacts the first metal layer there.

The solar cell according to the present invention, having a passivatedand large area metalized backside, which was reinforced chemically orgalvanically in the narrow finger areas and current collecting busbar orsoldering pad areas that were opened in the dielectric cover layer, hasin any case, in expedient embodiments, the following advantages comparedto the related art:

-   1.) In addition to the large area aluminum layer having local    contacts to the semiconductor surface, that was usual up to now, the    solar cell also has areas capable of being soldered (busbars or    soldering pads)-   2.) The chemically/galvanically reinforced (i.e. very conductive    fingers on the backside collect the current everywhere on the large    wafer area and conduct it on, in a low ohmic manner, to the busbars    or pads. The generated current, proceeding from the local contact    points out of the solar cell, has to cover only a small distance up    to the next plating finger in the metal layer sequence. Therefore,    this metal layer sequence is able to be made up of very thin metal    layers, such as a 0.1 μm nickel seed layer. Thus, it is able to be    produced, at moderate deposition rates, in so short a time that the    cells are not heated up too much.-   3.) Even after the finished production of the chemical or galvanic    reinforcement by the dielectric cover layer, the large area metal    layer sequence remains protected over a lifetime of 25 years from    chemical attack, such as corrosion in the module.-   4.) The thin metallization and the local subsequent reinforcement,    differently from the screen printing metallization of the backside    that was typical up to now, do not lead to wafer bending. This makes    it possible further to reduce the wafer thickness/cell thickness,    and thereby save costs for silicon.

In one embodiment that is technical and expedient from a cost point ofview, the second metal layer has at least one metal from the groupincluding Pd, Ni, Ag, Cu and Sn. In this instance, the second layer inparticular has a sequence of metal layers, particularly the layersequence Pd/Ni/Ag or Ni/Cu/Sn.

Moreover, it is provided that the second metal layer be produced by achemically or galvanically deposited reinforcement layer.

One additional design provides that the thin dielectric cover layer hasat least one material from the group including silicon oxide, siliconnitride, aluminum oxide, aluminum nitride and titanium oxide.

In a first variant the substrate involves a p-doped silicon substratehaving a phosphorus-doped emitter on the first main surface. In oneembodiment alternative to this, the semiconductor substrate is ann-doped silicon substrate having a boron or aluminum-doped p⁺ emitter onthe second main surface.

In particular, the first and second openings in the thin dielectriccover layer are formed by masked ion etching, especially in the samesystem in which the cover layer is produced. Alternatively to this itmay be provided that the first and second openings in the thindielectric cover layer are formed by laser ablation or by etching whileusing an etching paste that is applied using screen printing or inkjetpressure.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1 to 3 show an initial situation as well as a first and secondprocess section of the production of a specific embodiment of the solarcell according to the present invention, in perspective schematicrepresentations.

FIGS. 4A and 4B show schematic top views of two embodiments of a contactpattern of the solar cell.

FIGS. 5 and 6 show a third process section in schematic perspectiverepresentations.

DETAILED DESCRIPTION OF THE INVENTION

It remains open in all the figures whether a p-doped or an n-doped waferis involved. In the case of the n-material, the emitter may be situatedeither on the front side or the backside. In the latter case, as a rule,a doping for a front surface field (FSF) is produced on the front side.

The starting point of the following description is an almost completelyproduced solar cell that may be contacted on both sides on p- or n-dopedsilicon 1 (FIG. 1). Assume front side 2 a has already been processedcompletely, i.e. doped homogeneously or selectively, having apassivation/antireflection layer or layer sequence, and being providedwith a solderable contact grid. Backside 2 b may be undoped or maycontain a homogeneous doping 3, which represents an emitter or a BSF.

The surface is coated using chemical vapor deposition (CVD) or physicalvapor deposition (PVD, that is, vapor deposition or sputtering) using apassivating layer or layer sequence 4, selected and optimized withrespect to doping polarity and doping concentration, which has beenlocally removed (opened) using a technique known per se from the relatedart. The point grid thus created, in which the local contact openings 5are situated, goes by the layer resistance of the surface: in the caseof undoped surfaces (PERC cell) the distance apart of points D is lessthan for doped surfaces (PERT cell, that is, full surface doped cell).

The first process step according to the present invention begins withthe full-surface coating (known per se from the related art) using ametal layer sequence 6 (FIG. 2), either vapor deposition or sputteringtechnique being able to be used. In this context, for instance, aluminummay be involved, which is covered optionally in the same system using athin (not shown) nickel-containing seed layer for the later chemical orgalvanic reinforcement. As an alternative, of course, other metals ormetal layer sequences may be selected, e.g. titanium/palladium/silver orchrome-nickel/nickel/silver.

Finally, in all cases (in the same system, without interrupting thevacuum) the entire backside is covered with a thin dielectric thin layer7. In this context, all deposited layers lie both on passivating layersequence 4 and on the semiconductor surface exposed in local openings 5in the passivating layer. Cover layer 7 may be an oxide or a nitride ofthe aluminum or the silicon. It may be vapor-deposited or sputteredreactively from the metal target or deposited using RF sputtering fromthe dielectric target.

In the second step, using a suitable technique, the dielectric coverlayer is opened (FIG. 3) in the form of many, narrow lines 8 that arepreferably equidistant from and situated parallel to one another. Inthis context, for example, the following techniques may be used, thatare known per se from the related art: laser ablation, screen-printed orinkjet-printed etching paste, inkjet-masked wet chemical etching. Thedistance W of parallel openings 8 is typically of the order of magnitudeof 1 mm to 10 mm, preferably 2 mm to 5 mm. The width of the openings istypically about 100 μm to 1 mm, preferably 200 μm to 500 μm.

At 90° to the course of narrow openings 8, wider areas 9 a or 9 b areopened simultaneously, i.e. in the same opening step, in the dielectriccover layer, in which later solderable busbars and soldering pads are tobe produced. Busbar strips 9 a (FIG. 4A) may have any width desired thatis optimized for the soldering process. It might also be only 2 busbars.Pad areas 9 b (FIG. 4B) may be as many as desired and designed as big asdesired. They may be situated along 2 or 3 lines, depending on thenumber and the position of the busbars on the front side. In the thirdstep, the metal surface lying open in openings 8, 9 a, 9 b in the coverlayer is reinforced in a suitable chemical or galvanic depositingprocess using a very well conducting and well solderable metal layersequence (FIG. 5). In this context, depending on the exposed metal, forexample, palladium, nickel and silver or nickel, copper and tin may beinvolved. If aluminum having a nickel seed layer has been selected,nickel is deposited with a high probability in the first bath, which isthen still covered with a sufficient quantity of silver. In the samereinforcement process, the wider busbar areas or pad areas 9 a or 9 bare also postreinforced and made solderable 11 FIG. 6) using the samelayer sequence. The postreinforced narrow fingers 10 open out eitherdirectly into the reinforced busbar areas 11 a or soldering pad areas 11b, or into narrow connecting lines between the pads (cf. FIG. 4).

Incidentally, the execution of the method is not restricted to theabovementioned examples and emphasized aspects, but only by the range ofprotection of the appended claims.

What is claimed is:
 1. A solar cell, comprising: a semiconductorsubstrate made of silicon and having a first main surface used as anincident light side and a second main surface used as a backside; apassivating layer sequence situated on the second main surface, whereinthe passivating layer sequence has a plurality of island-type recessesin which passivating layer material has been completely removed; a thinfirst metal layer situated on the passivating layer sequence and in therecesses such that the first metal layer covers the entire backside ofthe substrate; a thin dielectric cover layer covering the first metallayer, wherein the thin dielectric cover layer has a first regulararrangement of narrow line-type openings and a second regulararrangement of one of wider line-type openings or extended island-typeopenings, the openings of the first and second regular arrangementextending through the dielectric layer and exposing portions of asurface of the first metal layer, wherein the first regular arrangementof openings and the second regular arrangement of openings are alignedsubstantially transversely to each other; and a conductive second metallayer which contacts, and is able to be soldered at, the portions of thesurface of the first metal layer exposed by the openings of the firstregular arrangement and the openings of the second regular arrangement.2. The solar cell as recited in claim 1, wherein the second metal layerincludes at least one of Ni, Cu and Sn.
 3. The solar cell as recited inclaim 2, wherein the second metal layer is made up of a sequence ofmetal layers including Ni, Cu, and Sn.
 4. The solar cell as recited inclaim 3, wherein the first metal layer includes at least one of Al andNi.
 5. The solar cell as recited in claim 4, wherein the first metallayer is made up of a sequence of metal layers including Al and Ni. 6.The solar cell as recited in claim 3, wherein the second metal layer hasone of a chemically or galvanically deposited reinforcement layer. 7.The solar cell as recited in claim 3, wherein the thin dielectric coverlayer has at least one of silicon oxide, silicon nitride, aluminumoxide, aluminum nitride and titanium oxide.
 8. The solar cell as recitedin claim 3, wherein the semiconductor substrate is a p-doped siliconsubstrate having a phosphorus-doped emitter on the first main surface.9. The solar cell as recited in claim 3, wherein the semiconductorsubstrate is an n-doped silicon substrate having one of a boron-doped oraluminum-doped p+-emitter on the second main surface.